The IntelliBus protocol first implemented in 1999 utilizes a Manchester (Bi-Phase) type signal encoded messaging format to transfer data over a network. The message format comprises a sync pattern plus a Manchester encoded 18-bit word. The sync pattern is used to identify the beginning of each 18 bit word, and the sync pattern identifies that word as either a command or data word. Traditionally sync patterns for Manchester encoded words in communication systems are 3 data bit-times wide. The sync patterns are invalid data patterns and were either 1½ bit periods constant high signal followed by 1½ bit periods constant low signal, or 1½ bit period constant low signal followed by 1½ bit periods constant high signal.
Current systems oversample the asynchronous data patterns and use algorithms to find the sync patterns in the Manchester encoded signals. However, the length of time from the center to the end of the sync pattern limits the amount of frequency deviation from the expected rate that a receiver can tolerate. Thus, the sync pattern itself is the weakest part of the message decoding, and therefore sets the maximum-amount of deviation permitted.
It is desirable to be able to receive and transmit Manchester encoded data words on a communication network asynchronously, meaning that no communication clock signal needs to be transmitted with the Manchester encoded data. This places the burden of recognizing the beginning of a data word, and correctly receiving each bit of the word, on each receiving node in the communication system even in the presence of distortion, or apparent distortion, of the received data signal. Apparent distortion can be caused by a mismatch of the frequencies of the expected transmit and receive frequencies in the communication system, or by duty cycle skew in the receiver or transmitter. Yet another form of distortion can be caused by undesirable interfering noise signals added to the desired data signal. Noise sources can be from reflections on the network cable, crosstalk between network cable pairs, or other forms of conducted or radiated noise signals.
In order to receive data words in the presence of a mismatch in expected data rate between a transmitter and receiver node, it is advantageous for the receiving node to oversample the received data signal and look for the crossings between the high and low portions of the received data signals. The receiver then makes a decision on where the beginning of the word is located, and whether the data bits are valid and should be saved to a register or other memory.
It is also desirable that the network signals not contain a DC component. This allows the network signals to be passed through transformers, capacitors, or radio links, as required.
Traditional sync patterns used in Manchester encoded communication systems limit the frequency deviation that can be tolerated between the transmitter and receiver. The present invention comprises an improved set of sync patterns that allow greater mismatch in data frequency between the transmitting and receiving network nodes for a given oversampling rate.